Cobalt alloys



AM MAAA. M v f 44 United States Patent ice 3,180,012 Patented Apr.27,1965

3,180,012 COBALT ALLOYS Gaylord Darrel Smith, Wilmington, Del., assignorto E. I. du Pont de Nemours and Company, WilmingtomDeL, a corporation ofDelaware No Drawing. Filed July 12, 1963, Ser. No. 294,753

a 4 Claims. (Cl. 29-4825) This invention relates to cobaltalloycompositions and to metal objects produced therefrom. Moreparticularly,

this invention relates to cobalt base alloy compositions alloys canpractically be utilized. Such materials would find use, for example, inthe fabrication of turbine blades and nozzles for jet engines, gasturbines, and the like, as well as in extrusion dies and forging andother tools for hot workingiof metals. It is known that improved hightemperature properties may result from elemental additions to cobalt ofrefractory metals, such as molybdenum and tungsten. It is also knownthat these refractory metals are very susceptible to high-temperatureoxidation and that either coatings or alloys must be perfected tomake'the refractory metal-containing alloys useful. Despite the factthat many alloys have been investigated, need remains for specificsuperior property combinations.

It is a primary object of the present invention to provide strong,tough, heat-resistant alloys consisting essentially of cobalt,molybdenum, and silicon in the propor tions by weight of S70% cobalt,2548% molybdenum, and 240% silicon and shaped objects comprising suchalloys.

Alloy objects of the invention are prepared either-by melting andcasting compositions consisting essentially of by weight usingtemperatures 400-800" F. above the solidus temperature or by firstprereacting such composi tions of cobalt, molybdenum, and silicon,reducing the reaction product to powder size and converting the powdersto objects by cold pressing followed by partial melt ing, by hotpressing at elevatedpressures, or by melting and casting. v

Alloys of cobalt-molybdenum-silicon falling within the compositionranges cited above are found to exhibit a number of outstanding andsurprising properties. These alloys are strong at room temperature andmaintain high transverse rupture strength at temperatures up ,to 2200"F. These alloys break with no measurable elongation below 1800 F. butcan fail plastically at higher temperatures. Furthermore, these alloysexhibit high impact strength and are resistant to thermal stress and todegradation by heat and air. These properties make these alloys suitablefor various critical service structural components such as gas turbineblades, forging dies, extrusion dies, gasoline and diesel exhaustvalves, and furnace fixtures. Shaped objects of these alloys can befabricated to close dimensional tolerances.

. oxidation resistance.

'tural materials.

bon, sulphur, boron, oxygen, and nitrogen, can be tol- Alloys outsidethe above-definedranges are found to be significantly deficient in oneor more of the above desired properties for the alloys of the invention.Alloys containing less than by weight molybdenum have strengths whichdiminish rapidly above 1600 F. It is found that alloys containing morethan 48% by weight molybdenum exhibit very poor mechanical properties.The use of less than 2% by weight silicon leads to highly inferioroxidation resistance at elevated temperatures. Alloys containing morethan 10% by weight silicon possess inferior mechanical properties.

Other elements may be added to the cobalt-molybdenum-silicon alloys ofthe invention provided they do not have a substantial adverse effectupon one or more of the properties: high transfer rupture strength atelevatedtemperatures; good impact strength; and excellent There, ofcourse, may be added such amounts of other elements Which will impart tothe alloys of the invention, in an anticipated manner, variousproperties that are desirable for special application with outsubstantially affecting their inherent attributes as struc- The presenceof nonmetals such as carerated in small amounts but the total should notbe al lowed to exceed about 0.1 Weight percent of the alloy composition.

Examination of alloys of the invention by metallographic techniquesshows that on the basis of micro structure the alloys consist of about20-85 volume percent of a Laves phase of the MgZn type structure andfrom about 1580 volume percent of a matrix composed of at least one orboth of the intermetallic compounds Co Si or COqMOs- Co Si or Co Mocoexist with the Laves phase in the liquidus-solidus temperature rangeof the composition of this invention. Total trace volume fractions ofsomething less than about 5 volume percent of the following phases maybe present in a microstructure of these alloys as impurities: Co, Co Mo,CoSi,

Mo Si, and Mo Si In preparing the compositions ofcobalt-molybdenumsilicon from which alloy objects of the invention areformed, it is preferred to use commercially pure elements. It is to beunderstood that minor changes in the relative proportions of theessential elements will occur when the untreated compositions areconverted to alloy objects due to the elimination of expendableimpurities in'the starting materials. However, for simplicity ofexpression and ease of understanding, the proportions of essential ele-'ments in the untreated compositions and in the. alloy compositionsand'shaped objects are hereafter referred to in terms of the samenumerical values. 7

The preferred mode offorming shaped objects comprising alloys of theinvention is to first premelt the 'cobalt -molybdenum-siliconcompositions, then reduce the alloy to a powder and. convert the powderto a shaped in steel or tungsten carbide-lined equipment to a fineparticle size, e.g., 95% of which will pass a 240 mesh screen. Thepowders are preferably stored in airtight containers maintained atslightly elevated temperatures in order to prevent moisture pickup untilused in producing shaped objects.

The powders are readily shaped, for example, by cold pressing in steeldies at pressures in a range of from about 3050 tons per square inch.The cold pressed object is then liquid phase bonded at temperaturesbetween 2100 F. and 2500 F. for a period of time ranging from less thanone minute :to as much as 60 minutes. This temperature range is betweenthe solidus and-liquidus line for these alloy compositions. Inert gas,hydrogen, or vacuum furnace atmospheres are satisfactory and will yielddense, bright shaped objects.

ten pounds were converted to alloys of the invention by arc-melting. Thearc-melter was designed with a tungsten electrode and a deep boat-shapedcopper hearth to minimize contamination and weight losses. To insurehomogeneity, each sample was arc-melted at least four times and theresulting alloy buttons were examined metaliographically for propertycharacterization. The results of examination on these alloys arereported in Table I below including the v/o of Laves phase determined,the Knoop microhardness of the Laves phase, the average grain size, thetype of cast structure, and crystal structure of the matrix phase. Thealloys all exhibit a Laves phase content between and 85 v/o. Knoopmicrohardness values of the Laves phase present in the microstructure ofthese alloy ranges from 900 to 1443.

Table 1 Composition (w/o) Laves Laves phase Lavesphase CompositionCrystal phase (Km micro- (grain size, structure structure (v/o)hardness) d in type of matrix 00 Mo Si microns phase 70 28 2 20 1, 039 5Eutectic 65 10 50 1, 039 8 Dendritic. C07M0fl 65 29 6 50 1, 030 15 do 6533 2 50 1, 039 35 Eutectic+ dendritie" 60 10 58 1, 220 10 Dendritic- CoMo 60 34 6 65 950 25 Dendtitic-jslight eutectic C0 Mor 60 38 2 50 1, 03925 Eutectic-{- dendritic. 55 10 80 1, 231 Dendritic- (30281 39 6 78 1,200 30 Coarse den- 'e CmMo 55 41 4 8d 954 80 Coarse dendrites Co Mo 5543 2 75 900 10 Fine grain two phase..- 50 40 6 76 1, 432 20 Coarsedendrites C01Moa 50 48 2 76 1, 443 15 Fine grain two phase..- CO7MO5Shaped objects can also be made from the powders by hot pressing thepowder in graphite dies at temperatures between 2100 F. and 2400" F. at1000 pounds per square inch or higher. Atmosphere control in thisoperation is not critical. Soaking time at the operating temperature isdependent upon the mass of the object but will usually be limited totimes in the range of about 5-20 minutes.

Shaped objects can also be made from the powders by melt casting them inair, vacuum, or inert atmospheres using molds of graphite or stableoxides, such as MgO, Z102, or T1102. Graphite crucibles can be usedwhere the cast objects may be later ground to remove the surfacematerial contaminated by the graphite. In forming shaped objects bycasting the powders, it is found that temperatures in excess of [2750 F.are preferred to obtain good fluidity.

A better understanding of the invention will be gained from thefollowing working examples. In these examples, the starting materialsemployed were of conventional commercial purity. The abbreviations w/oand v/o represent percent by weigh and percent by volume, respectively.

EXAMPLE 1 A series of compositions of cobalt-molybdenum-silicon fallingwithin the cited ranges weighing between one and A representative groupof the alloy buttons of Table I were then separately ground to powder byjaw crushing to -4 mesh followed by ball milling, either dry or withbenzene, in a four-quart capacity steel mill (8 inches 'diameteryusingtungsten carbide inserts 4" x /2" x /2"). The mill was run at 60.r.p.m.from one hour to as much as twenty-four hours to obtain at least -230mesh powder.

Solid bars GA x /2" x 2") were then made by cold pressing thecompositions in steel dies at pressures in the range of 30 to 50 tonsper square inch followed by liquid phase bonding at temperatures between2282 and 2462 F. for approximately ten minutes. Either vacuum orcontrolled atmosphere furnaces were used to prevent oxidation of theparticulate material prior to densification.

These bars were then tested to determine the following properties: Rhardness; transverse rupture strengths at various temperatures; impactstrength as determined using unnotched Izod test specimens; oxidationresistance at various temperatures; and thermal shock. The results ofthese tests on the shaped objects are reported in Table 11 belowtogether with information on the liquid phase sintering temperature inpreparing the bars, the shrinkage occurring during the sintering step;and the bulk density of the shaped objects.

Table II Composition Liquid phase Shrink. Transverse rupture strength,1,000 p.s.i. Impact (w/o) sinter. during strength unliquid Bulk RAnotched Izod phase sint. dens. Hardness 70 F. 932 F. 1472 F. 1832 F.2192 F. test (ft.lbs./ Mo S1 F. 0. (percent) (g./cc.) (21 C.) (500 C.)(800 C.) (1000 C.) (1200 0.) sq. in.)

70 28 2 2, 372 1, 300 cast 9. 08 80. 5 165. 0 154. 0 128. 0 52. 5 55. 065 25 2, 174 l", 190 21. 5 7. 55 77. 0 78. 8 79. 9 68. 0 22. 6 27. 8 6030 10 2, 120 l, 100 30. 5 8. 34 83. 4 85. 4 102.0 64. 1 16. 5 29. 7 5535 10 2, 200 1, 205 30. 5 8. 40 82. 0 94. 7 101. 0 99. 0 18. 3 28. 0 5539 6 2, 282 1, 250 27. 5 8. 79 60. 9 53. 8 36. 3 24. 4 55 41 4 2, 462 1,350 31. 5 8. 63 82 53 54. 2 65. 4 72. 2 24. 4 50 44 6 2, 372 1, 300 33.5 8. 77 78 53 60. 6 65 68. 5 24. 4 50 48 2 2, 372 1, 300 35. 0 9. 09 8363. 1 65. 9 56. 1 70. 7 26. 0

Composition (w/o) Oxidation resistance Shock cycles from 400 F. to

(mgJinP/IOO hrs.) temperature indicated Go M0 Si 1112 F 1472 F. 1832 F.1472'F. 1652 F. 1832 (600 C (800 C.) (1000 0.) (800 0.) (900 C.) (1000C) 70 28 2 -77. 0 r 65 10 0 0 0. 0 25 25 *50 *Craok on 5th cycle-noother efiect. 55 35 10 +1. 2 130. 0 25 25 50 55 39 6 +2. 5 3. 9 84. 0 25*25 50 *Slight crack 3rd cyc1eno other effect. 55 41 4 +2. 6 +5. 2 -77.0 25 25 50 50 44 M 6 +1.0 7.1 -77. 0 25 25 50 60 48 f3} 2 +0. 6 -2. 684. 0 25 25 *501 *Crack on 17th cycle-no other effect.

It will be noted from the above reported results that the R hardnessnumber ranges from R 77 to R 83.4. The room temperature transverserupture strengths range from about 53,000 to 165,000 lbs/sq. in.; fromabout 56,000 to 128,000 lbs/sq. in. at 1472 F. (800 C.); as high as72,000 lbs/sq. in. at 1832 F. (1000 C.); and as high as 17,000 lbs/sq.in. at 2192 F. (1200 C.). Impact strengths range from 24 to 55 ft.lbs/sq. in. The oxidation resistance is excellent at temperatures below1472 F. (800 C.). At 1832 F. (1000 C.) the oxidation rate has increasedto as much as 130 milligrams weight loss per sq. in. per 100 hours. Thisrate of oxidation is, of course, still acceptable for selectedapplications.

It will be noted that an important characteristic of these shapedobjects is their resistance to deformation at all temperatures belowapproximately 1800 F. All samples tested failed without yielding below1800 F. Samples tested in transverse rupture at 2192" F. (1200 C.)failed plastically.

EXAMPLE 2 position into shaped objects of the invention and furtherillustrates the unique properties and outstanding utility .of suchshaped objects.

was made generally in accordance with the procedure shown in Example 1above;

From a part of this sample of powder, eight test bars A" x /2" x 2")were hot pressed in graphite molds at 2192 F. for five minutes underapproximately 3000 lbs/sq. in. The average R hardness number for thesespecimens was 82. The room temperature transverse rupture strength forthe specimens ranged from 83,000 to 95,000 lbs/sq. in. The averageimpact strength was 18 ft. lbs/sq. in. All the bars passed thermal shockcycle testing without cracking.

From the balance of the sample of powder, four discs thick x 3"diameter) were hot pressed in a graphite mold at a temperature or" 2192F. for 20 minutes under 3000 lbs./ sq. in. The resultant discs weredetermined to be flaw-free by ultrasonic testing techniques. The discswere determined to have an average bulk density of 8.10 grams/ cubiccentimeter and R hardness of 82. The discs were then machined byelcctrospark machining to make extrusion dies for extruding aluminum.Two of these dies were made to produce 0.850" round solid aluminum rods.This pair ofdies successfully extruded twelve billets (30 lbs. each) ofAA6063 aluminum alloy preheated from 800 :to 850 F. at 300 ft./lmin.with a' good finish on the aluminum. This compares to a normalproduction speed of ft./min. under similar conditions with conventionaldie materials.

. EXAMPLE 3 The following illustrates melting and casting as a means forconverting untreated compositions of cobalt-molybdenum-silicon intoshaped objects of the invention. The properties of these alloy objectsare reported in Table III below.

Table' III Ultimate tensile strength (p.s.i.) 131,000 Youngs modulus(X10 p.s.i.) 35.4 Charpy V-notch impact strength (ft. lbs./ sq.

in.) 1.25 R (as-cast) 75-78 R (after heat treatment at 1600 F./l5 hours)80.5 Specific heat at room temperature (gram-cal./

C.) .127 Specific heat at 325 F. (gram-cal./- C.) .111

Thermal conductivity (gram-cal./sec./cm.

C.) .O7-.08 Coefiicient of expansion 10 in./in./ F.)

(RT. to 1500 F.) .4 10.4

The alloy objects of the invention can be machined to close toleranceswith tungsten carbide tooling procedures or by grinding, electrosparkmachining, or ultrasonic machining. Alloy compositions containing lessthan 40% by weight molybdenum and less than 8% by weight silicon arereadily machined with tungsten carbide tooling. The alloy compositionscontaining more than 40% by Weight molybdenum and more than 8% by weightsilicon are best machined to finished dimension by either electrosparkor ultrasonic machining techniques. If desired, the

alloy objects of the invention may be subjected to various metallurgicalprocesses well known in the art; such as heat treatment, hot rolling,extrusion, or the like to develop improved properties in an anticipatedmanner.

As many apparently widely different embodiments of this invention may bemade Without departing from the spirit and scope thereof, it is to beunderstood that this 7 invention is not limited to the particularembodiments described hereinabove except as defined in the appendedclaims.

I claim:

1. An alloy composition consisting essentially of cobalt,

' reacted powder having all particles pass a 240 mesh 3. A shaped objectformed of the powder metallurgy l5 composition of claim 2 sintered to asubstantially homogeneous composite.

4. A metallurgy composition comprising a fine prescreen, and consistingessentially of cobalt, molybdenum,

and silicon in the proportions by weight of 5070% cobalt, 2548%molybdenum, and 2-10% silicon.

References Cited by the Examiner UNITED STATES PATENTS 1,710,445 4/29Becket 75-470 1,949,313 2/34 Koster 75170 2,100,218 11/37 Kelley 751702,770,029 11/56 Weltz 7 517 6 XR FOREIGN PATENTS 407,017 12/24 Germany.

CARL D. QUARFORTH, Primary Examiner.

REUBEN EPSTEIN, Examiner.

1. AN ALLOY COMPOSITION CONSITING ESSENTIALLY OF COBALT, MOLBDENUM, ANDSILICON IN THE PROPORTIONS BY WEIGHT OF 50-70% COBALT, 25-48%MOLYBDENUM, AND 2-10% SILICON.